††††††††† The human
respiratory system consists of the lungs and tubes associated with the
lungs. It is located in the thorax or chest. The thorax is surrounded by
the ribs. The diaphragm forms the base of the thorax.

††††††† Contractions of the diaphragm and the intercostals
muscle change the size of the thorax and, thus, cause air to move in and
out of the lungs.

††††††† The main job of the respiratory system is to get oxygen into
the body and get waste gases out of the body. It is the function of the
respiratory system to transport gases to and from the circulatory system.

As air passes
through the nasal cavities it is warmed and humidified, so that air that
reaches the lungs is warmed and moist.†
The Nasal airways are lined with cilia and kept moist by mucous
secretions. The combination of cilia and mucous helps to filter out solid
particles from the air an Warm and moisten the air, which prevents damage to
the delicate tissues that form the Respiratory System. The moisture in the nose
helps to heat and humidify the air, increasing the amount of water vapour the
air entering the lungs contains. This helps to keep the air entering the nose
from drying out the lungs and other parts of our respiratory system. When air
enters the respiratory system through the mouth, much less filtering is
done. It is generally better to take in air through the nose.

††††††††† The pharynx is also called the throat.
As we saw in the digestive system, the epiglottis closes off the trachea
when we swallow. Below the epiglottis is the larynx or voice
box. This contains 2 vocal cords, which vibrate when air passes by
them. With our tongue and lips we convert these vibrations into speech.
The area at the top of the trachea, which contains the larynx, is called
the glottis.

††††††† The
trachea or windpipe is made of muscle and elastic fibres with
rings of cartilage. The cartilage prevents the tubes of the trachea from
collapsing. The trachea is divided or branched into bronchi and
then into smaller bronchioles. The bronchioles branch off into alveoli.
The alveoli will be discussed later.

†† †††

††††††† These
tubes are lined with mucous-secreting cells and tiny hairs called cilia.
The mucous traps bacteria, dust and viruses. The cilia beat and create an
upward current. This moves the mucous up and into the oesophagus. Here it goes
to the stomach. When we clear our throats we force the mucous away from our
vocal cords. This is often called coughing. It is used to get rid of irritants
and excess mucous from our respiratory system.

††††††††† The lungs are spongy structure where the
exchange of gases takes place. Each lung is surrounded by a pair of pleural
membranes. Between the membranes is pleural fluid, which reduces
friction while breathing. The bronchi are divided into about a million
bronchioles. The ends of the bronchioles are hollow air sacs called alveoli.
There are over 700 million alveoli in the lungs. This greatly increases the
surface area through which gas exchange occurs. Surrounding the alveoli are capillaries.
The lungs give up their oxygen to the capillaries through the alveoli.
Likewise, carbon dioxide is taken from the capillaries and into the alveoli.

††††††††† Body cells use the
inhaled oxygen gotten from the alveoli of the lungs. In turn, they produce
carbon dioxide and water, which is taken to the alveoli and then exhaled. These
exchanges occur as a result of diffusion. In each case the materials move from
an area of high concentration to an area of lower concentration.

† †††††††††††††††

Here, the blood flows from the heart to
the lungs.†† ††††††††††† Here the blood flows from the lungs to the body cells.
The alveoli take in the water and the carbon dioxide††††††††††† The body cells take in the oxygen and give off the and
give off the oxygen.††††††††††††††† ††††††††††††††††††††††† ††††††††††† carbon dioxide and the water.

††††††† The
alveoli are well suited for the important job they have. There are about
300,000,000 alveoli per lung! That means there is a great surface area for gas
exchange. Also, the walls of the alveoli as well as the capillaries are very
thin so that the gases can diffuse readily.

††††††† When
the blood picks up the diffused gases the gases are carried to their
destinations. Most of the oxygen is carried by the haemoglobin in the
red blood cells with only a small % dissolved in the plasma. Carbon dioxide and
water are carried in the plasma of the blood.

The
following chart compares the content of air before as it is inhaled (Inspired
Air) and as it is exhaled (Expired Air).

Inspiration or inhalation
is said to be an active process because it involves muscle contraction. The diaphragm and intercostal muscles
contract. The contracting diaphragm flattens and stretches the elastic lungs
downward. The contracting intercostals pull the ribcage up and out
causing the elastic lungs to stretch. The expanding lungs cause the air inside
to expand (a gas will always fill its container). The expansion of air causes a
drop in air pressure in the lungs. The air in the lungs is at a lower pressure
than the air outside. Air flows from higher to lower pressure so air flows into
the lungs from outside.

Expiration or exhalation is said to be a passive
process because it does not involve muscle contraction. The diaphragm and
the intercostal muscles relax. The deforming force on the elastic lungs has
been removed. The lungs recoil elastically reducing their volume Ė a passive
process. The volume of air in the lungs decreases causing an increase in
the air pressure. The air in the lungs is at a higher pressure than the air
outside. Air flows from higher to lower pressure so the air flows out of the
lungs. The elastic recoil of the lungs pulls up the adhering diaphragm and
drags in the adhering ribcage.

Breathing is normally under unconscious control.
We donít have to think about breathing. Exercise increases the rate of
breathing. The brain detects a large increase in carbon dioxide and
increases the rate of breathing. Now, exhalation, which is normally passive,
becomes active. Other times when we control our breathing rate is in speaking,
singing, or swimming.

Breathing is always controlled by the brainís detection of
carbon dioxide in the blood. When carbon dioxide is in the blood the pH of the
blood is slightly lowered. The brain detects this slight drop and sends
impulses to the diaphragm and intercostal muscles. Thus, our breathing
mechanism is controlled by rising levels of carbon dioxide, not low levels of
oxygen. Just as the level of carbon dioxide controls the stomata opening in
leaves it also controls our breathing.

Asthma is a breathing disorder. Its symptoms include
coughing, wheezing, tightness of chest and breathlessness. It is caused by an allergic
reaction to materials in the environment such as pollen, cigarette smoke,
house dust and pet dander. More recently scientists have found a link between stress
and anxiety with the onset of asthma.

Asthma is a chronic ailment in which
inflammation of the airways, or bronchi, affects the way air enters and leaves
the lungs, thereby disrupting breathing. When allergens or irritants come into
contact with the inflamed airways, the already sensitive airways tighten and
narrow, making it difficult for the person to breathe. Progressively severe
symptoms can lead to an asthma attack. In asthma attacks, the overproduction of
mucus lining the airways further narrows the airways, limiting oxygen intake
and making it more difficult to breathe.

To prevent asthma the allergen must be
identified and avoided. Also, in the case of stress, the stress must be
alleviated.

The treatment of asthma is usually by Inhalers.
These devices (sometimes called 'puffers') contain a gas that will propel the
correct dose of medication when the top is pressed down. This is inhaled into
the airways. There are two basic
categories of inhaler medicines that are used for asthma: relievers - which
treat the symptoms and preventers - which can prevent the symptoms.